1. Field of the Invention
The present invention relates to a cathode that may be incorporated into organic light-emitting devices, display panels, organic transistors, and organic solid state lasers.
2. Description of the Related Art
Organic light-emitting devices (OLEDs) have recently become a prime focus of numerous researchers because of their relative simplicity of fabrication, large viewing angle, ultra-thin structure, mechanical flexibility, light weight, and faster response time. In particular, OLEDs are being investigated as candidates for commercial display applications, such as ultra-thin flat panel displays (FPDs), roll-up displays, and head-mounted displays, such as virtual reality and cockpit displays. The utility of OLEDs is expected to be especially important in FPDs incorporated in high definition televisions, personal computers, and portable computers.
A cross-section of a conventional bilayer (two organic layers) OLED 10 structure is shown in FIG. 1. The operation of the OLED 10 can be described as follows: upon the application of a voltage, holes (represented by open ovals) are injected from the anode 20 into the highest occupied molecular orbital (HOMO) of the molecules of a first organic layer 30, called the hole transport layer (HTL) 30, and electrons (represented by closed ovals) are injected from the cathode 40 into the lowest unoccupied molecular orbital (LUMO) of the molecules of a second organic layer 50, called the electron transport layer (ETL) 50. The charges drift under the influence of the external field and recombine in the emitting layer 60, which can be the HTL 30 or the ETL 50, thereby generating excited molecules. Some of the excited molecules decay radiatively, thus releasing light (represented by arrows). The materials forming HTL 30 and the ETL 50 are thus electroluminescent organic (ELO) materials. An example of a material for HTL 30 is TPD, the structure of which is shown in FIG. 2(A), and an example of a material for ETL 50 is 8-tris-hydroxyquinoline (Alq3), the structure of which is shown in FIG. 2(B).
One of the electrodes, for example the anode 20, can be transparent to allow the transmission of light to the outside environment so that a viewer can see it. Due to its relatively high transparency to visible light and its relatively good electrical conductivity, indium tin oxide (ITO) can be used as the material for the anode 20. A substrate 70, for example made of glass, can support the anode 20. The material of the cathode 40 is often a metal such as Al or Mg, although other variances have been used as discussed below.
In general, organic materials have a higher hole mobility than electron mobility. This relatively high hole mobility, as well as the presence of a high barrier ("PHgr"e) for electron injection at the cathode-organic layer interface, lead to an imbalance between the hole charge density and the electron charge density near the interface of the two organic layers. This behavior has a negative effect on the device external quantum efficiency, which is defined as the ratio of the number of photons collected (for example measured with a calibrated silicon photodetector) in the forward direction to the number of charges injected in the device.
One way to enhance the external quantum efficiency is to increase the number of injected electrons. This can be achieved by decreasing the barrier height between the work function of the metal cathode and the LUMO of the ETL. To that end, OLED cathodes based on metals with a relatively low work function such as lithium, calcium, or magnesium, are used and show higher external quantum efficiency than similar devices with cathodes such as aluminum (Al), copper, or silver. However, the major drawback of using low work function metals is their readily reactive nature, especially in air atmosphere, which results in unreliable OLEDs.
More environmentally stable cathodes such as Al are sometimes used. Aluminum is cheaper, more abundant, relatively resistant to full oxidation and corrosion, when exposed to atmospheric conditions, than either calcium or magnesium. Moreover, the compatibility of Al with silicon microelectronic circuits has made it a material of choice for micro-pixel OLEDs displays driven by thin film transistor or complimentary metal-oxide-semiconductor circuits. However, due to the high work function of Al, OLEDs with Al cathodes are inefficient, and their light output, at a given voltage, is an order of magnitude less than OLEDs with reactive metal cathodes.
A thin insulating layer, such as lithium fluoride (LiF), cesium fluoride (CsF), sodium fluoride (NaF), silicon dioxide, deposited between the Al cathode 30 and the organic layer 40 as a buffer layer has also been used and has lead to some improvements in performance. However, the deposition of a buffer layer requires very careful thickness control. Any thickness variation over the active area of the device leads to uneven electric field distribution, which results in nonuniform brightness, an unacceptable feature in display applications.
Cathodes of Alxe2x80x94Li alloy (a two-metal alloy) have also been tried since they do not require an insulating buffer layer. However, these cathodes are not very reproducible, mainly due to the strict Li content that must be maintained at 0.1% for optimum performance.
Although the devices have shown some progress in the reliability of OLEDs, in terms of operational lifetime, higher operational lifetime are desirable. Furthermore, there is a need for an OLED having enhanced external quantum efficiency and injected electrons densities, and at the same time being reliable, stable, not requiring a buffer layer and having an increased efficiency, reproducibility, and lifetime.
Examples of light-emitting devices are disclosed in U.S. Pat. No. 5,399,502; in Tang et al, xe2x80x9cOrganic Electroluminescent diodesxe2x80x9d Appl. Phys. Lett. 51 (12) 1987; and in Baigent et al, xe2x80x9cConjugated Polymer Light-emitting Diodes on Silicon Substratesxe2x80x9d Appl. Phys. Lett. 65 (21) 1994; the entire content of these three references being hereby incorporated by reference.
Accordingly, an object of the present invention is to provide a cathode having an increased injected electrons densities; and to provide a method for making the same.
Another object of the present invention is to provide an OLED having enhanced external quantum efficiency and injected electrons densities; and to provide a method for making the same.
Yet another object of the present invention is to provide an OLED that is reliable, stable, that does not require a buffer layer; and to provide a method for making the same.
A further object of the present invention is to provide an OLED with an increased efficiency, reproducibility, and lifetime; and to provide a method for making the same.
Another object of the present invention is to provide a display device with OLEDs having increased performance; the display being incorporated, for example, in FPDs for high definition televisions, personal computers, and portable computers; in roll-up displays; or in head-mounted displays, such as virtual reality and cockpit displays.
A further object of the present invention is to provide an organic transistor with a cathode having an increased injected electrons densities.
It is a further object of the present invention to provide an organic solid state laser with a cathode having an increased injected electrons densities.
In a first embodiment, the present invention provides a device including a layer of organic material and a cathode in contact with the organic material layer and including a mixture of a metal and an insulator. The mixture can be either an alloy, a composite, or a combination of both. The metal can be for example Al, Mg, silver (Ag), gold (Au), copper (Cu), nickle (Ni), iron (Fe), chrominum (Cr), indium (In), calcium (Ca), or a combination thereof. The insulator can be an inorganic insulator such as but not limited to alkali, and alkaline compounds, e.g.,: lithium fluoride (LiF), cesium fluoride (CsF), sodium fluoride (NaF), lithium bromide (LiBr), cesium bromide (CsBr), sodium bromide (NaBr), calcium fluoride (CaF2), magnesium fluoride (MgF2), berilium fluoride (BeF2), lithium oxides (Li2O), cesium oxide (Cs2O), rubidium chloride (RbCl), sobium chloride (NaCl), lithium borate (LiBO2), lithium iodide (LiI), cesium iodide (CsI), potasium silicate (K2SiO3), and combination thereof.
In a second embodiment, the present invention provides an OLED including an emitting layer of organic material; a cathode in contact with the emitting layer and including a mixture of a metal and an insulator; and an anode in contact with the emitting layer. The mixture can be either an alloy, a composite, or a combination of both. The metal can be a high work function metal, or can be Al, Mg, Ni, Fe, Cr, In, Ca, Au, Ag, and combination thereof. The insulator can be an inorganic insulator such as but not limited to alkali, and alkaline compounds, e.g.,: lithium fluoride (LiF), cesium fluoride (CsF), sodium fluoride (NaF), lithium bromide (LiBr), cesium bromide (CsBr), sodium bromide (NaBr), calcium fluoride (CaF2), magnesium fluoride (MgF2), berilium fluoride (BeF2), lithium oxides (Li2O), cesium oxide (Cs2O), rubidium chloride (RbCl), sobium chloride (NaCl), lithium borate (LiBO2), lithium iodide (LiI), cesium iodide (CsI), potasium silicate (K2SiO3), and combination therof.
In a third embodiment, the OLED includes a metal capping layer in contact with the composite cathode. The metal capping layer can be made of Al, Cr, Ag, Au or combination.
In a fourth embodiment, the OLED includes a substrate in contact with an anode. The substrate can be made of a ceramic such as glass or silicon, plastic, or metal.
In a fifth embodiment, the emitting layer includes a multi-layer having a hole transport layer and an electron transport layer. The electron transport layer can be made of Alq3, and the hole transport layer can be made of TPD. The emitting layer can also be a single or multilayer with organic dye as dopant.
In a sixth embodiment, the present invention provides an organic transistor including a cathode in contact with an organic material layer and including a mixture of a metal and an insulator. The mixture can be either an alloy, a composite, or a combination of both. The metal can be for example Al, Mg, Ni, Fe, Cr, In, Ca, Au, Ag, and combination thereof. The insulator can be an inorganic insulator such as but not limited to alkali, and alkaline compounds, e.g.,: lithium fluoride (LiF), cesium fluoride (CsF), sodium fluoride (NaF), lithium bromide (LiBr), cesium bromide (CsBr), sodium bromide (NaBr), calcium fluoride (CaF2), magnesium fluoride (MgF2), berilium fluoride (BeF2), lithium oxides (Li2O), cesium oxide (Cs2O), rubidium chloride (RbCl), sobium chloride (NaCl), lithium borate (LiBO2), lithium iodide (LiI), cesium iodide (CsI), potasium silicate (K2SiO3), and combination thereof.
In a seventh embodiment, the present invention provides an organic solid state laser including a cathode in contact with an organic material layer and including a mixture of a metal and an insulator. The mixture can be either an alloy, a composite, or a combination of both. The metal can be for example Al, Mg, Ni, Fe, Cr, In, Ca, Au, Ag, and combination thereof. The insulator can be an inorganic insulator such as but not limited to alkali, and alkaline compounds, e.g.,: lithium fluoride (LiF), cesium fluoride (CsF), sodium fluoride (NaF), lithium bromide (LiBr), cesium bromide (CsBr), sodium bromide (NaBr), calcium fluoride (CaF2), magnesium fluoride (MgF2), berilium fluoride (BeF2), lithium oxides (Li2O), cesium oxide (Cs2O), rubidium chloride (RbCl), sobium chloride (NaCl), lithium borate (LiBO2), lithium iodide (LiI), cesium iodide (CsI), potasium silicate (K2SiO3), and combination thereof.
The above embodiments also apply to a single or multi-layer: polymer OLED as well as molecular-polymer blend OLED, and organic-inorganic hybrid OLED.